Glass sheets and other substrates can be coated with a stack of transparent, metal-containing films to vary the optical properties of the coated substrates. Particularly desirable are coatings characterized by their ability to readily transmit visible light while minimizing the transmittance of other wavelengths of radiation, especially radiation in the infrared spectrum. These characteristics are useful for minimizing radiative heat transfer without impairing visible transmission. Coated glass of this nature is useful as architectural glass and as automotive glass.
Coatings having the characteristics of high visible transmittance and low emissivity typically include one or more infrared-reflective films and two or more antireflective transparent dielectric films. The infrared-reflective films, which are typically conductive metals such as silver, gold, or copper, reduce the transmission of radiant heat through the coating. The transparent dielectric films are used primarily to reduce visible reflection, to provide mechanical and chemical protection for the sensitive infrared-reflective films, and to control other optical coating properties, such as color. Commonly used transparent dielectrics include oxides of zinc, tin, and titanium, as well as nitrides of silicon, chromium, zirconium, and titanium. Low-emissivity coatings are commonly deposited on glass sheets through the use of well known magnetron sputtering techniques.
It is often necessary to heat coated glass sheets to temperatures at or near the melting point of glass to temper the glass or to enable it to be bent into desired shapes, such as curved automobile windshields. Tempering is important for glass used in automobile windows, and particularly for glass used in automobile windshields. Upon breaking, tempered glass desirably exhibits a break pattern in which the glass shatters into a great many small pieces, rather than into large dangerous shards. During tempering, coated glass is typically subjected to elevated temperatures on the order of about 700 degrees C. Moreover, coated glass often must be able to withstand such temperatures for substantial periods of time. Film stacks employing silver as the infrared-reflective film often cannot withstand such high temperature processing without some deterioration of the silver film.
To avoid this problem, glass sheets can be heated and bent or tempered before they are coated. The desired films can then be applied after heating. However, this procedure tends to be complicated and costly and, more problematically, may produce non-uniform coatings.
Another reported method for protecting a reflective silver film from deterioration at high temperatures involves sandwiching the silver between protective films of an oxidizable and/or nitridable metal (e.g., titanium). The protective films are thick enough and reactive enough that when the coated glass is heated to high temperatures, these films capture oxygen and/or nitrogen that might otherwise reach and react with the silver. During heat treatment, the atoms in the originally flat silver film become particularly mobile, and even more so after being activated by presence of oxygen. As a result, the silver may begin growing hillocks, which may ultimately lead to the formation of isolated metal islands (clusters). This will generally create an unacceptable amount of visible haze, reduce infrared reflection, and increase emissivity. Reference is made to U.S. Pat. No. 4,790,922, (Huffer et al.), U.S. Pat. No. 4,806,220 (Finley), and U.S. Pat. No. 3,962,488 (Gillery), the entire teachings of each of which are incorporated herein by reference.
It is also known to provide a single protective titanium layer directly over an infrared-reflective silver film to protect the silver film during deposition of a subsequent oxide layer. Protective titanium layers have been found to impart excellent scratch resistance in the low-emissivity coatings into which they are incorporated. However, low-emissivity coatings having titanium protective layers tend to change noticeably in color (i.e., they tend to color shift) when tempered. As a result, glass bearing such a coating tends to exhibit a noticeably different color before being tempered than it does after being tempered. This can have undesirable consequences for quality control, as the final appearance of the product tends to show up only after tempering, which may be performed at a separate location and at a later time. This creates difficulties for feedback to control the production process.
To ensure that tempered and non-tempered panes have uniform appearance, the temperable coating is designed to have substantially the same appearance following tempering as the normal appearance of the non-temperable coating. Temperable coatings are generally not used without first being tempered, as these coatings may only reach their desired appearance (i.e., their final specification) after they have been tempered. It is preferable to provide coatings that change as little as possible in color and other properties during tempering and other heat treatments.
U.S. Pat. Nos. 6,060,178 and 6,231,999 (both issued to Krisko), the entire contents of each of which are incorporated herein by reference, disclose low-emissivity coatings that employ niobium protective layers. Low-emissivity coatings having niobium protective layers are particularly advantageous in that they show minimal shifts in properties (e.g., color shift) when tempered or otherwise heat treated. However, it has been discovered that low-emissivity coatings having niobium protective layers are less scratch resistant than otherwise equivalent coatings having titanium protective layers.
It would be desirable to provide a protective layer that imparts in low-emissivity coatings both scratch resistance and resistance to the color shifting that can occur during tempering and other heat treatments. It would be particularly desirable to provide a protective layer that imparts these characteristics, yet can be incorporated into low-emissivity coatings at an affordable cost.